Transducer



July 19, 193s.

E. c. GUEDQN TRANSDUCER Filed July 31. 1935 IN VEA/'ron C. Guedon i, 22 Emile .n a A Patented July 19, 1938 PATENT OFFICE 2,124,296 TRANSDUGER :Emile vC. Guedon, Audubon, N.

Radio Corporation of America,

of lela'ware J., assigner to a corporation Y Appncatin july 31, 1935, serial No. 34,016V

10i Claims.

The present invention relatesto transducers or translating devices for converting or translating electrical signal currents into corresponding mechanical Vmovement of an armature member and for converting or translating mechanical movement of saidarmature member into corresponding electrical currents.

More particularly, the invention relates to transducersof the electromagnetic pickup type employed in phonographrecording and reproducing systems and comprising a vibratory armature adaptedto carry a stylus or needle, an electric pickup coil surrounding the armature and a permanent magnet associated with the armature and the coil to provide exciting flux.

The invention has for its primary object to provide an improved and simplified construction in a transducer device of the character referred *to having operating elements which may be more rapidly and easily assembled and adjusted in predetermined relation to each other and which may withstand more severe handling in shipment land'in servicing than has heretofore been possible in like devices.

It has been the practice heretofore in transducers of the electric pickup type to provide a pair of pole pieces for the permanent magnet having an air gap maintained constant by a nonmagnetic bridge piece connecting the pole pieces. The magnet itself is usually removable from the pole pieces, being held thereon merely'by aspring tension or by magnetic attraction alone. Because of this arrangement the magnetic circuit wasoften momentarily broken when the magnet was moved by vibration or by accident in servicing the device, with the result that the magnetic field thereafter was greatly reduced in strength and required recharging of the magnet before efcient operation could be obtained.

It is, therefore, a further objectof the present invention to provide an improved magnet and pole piece construction in a transducer device of the electro-magnetic pickupI type, wherein the air gap is maintained at a permanent predetermined width-by the magnet itself acting as a yoke, permanently welded to the pole piece.

It is also a still further object of the invention to provide an arrangement of parts in a transducer device of the electro-magnetic pickup type which renders such a device compact in construcvtion and highly efficient electrically in producing relatively high output potentials in response to normal vibratory movement of the armature.

The invention will, however, be better understood from the following description when con- -vide a good magnetic joint at sidered in connection with the accompanying drawing and its scope will be Vpointed out in the appended claims. V

In the drawing, Fig. 1 is a front view on a greatly enlarged scale of an electro-magnetic pickup device or transducer embodying the invention, with a cover removed to expose the in- I terior mechanism;

f Fig. 2 is a cross `sectional view of the device shown in Fig. 1 and on the same scale, taken on line 2-2 vand with the cover in place;

Fig. 3 is a bottom view of the device shown in Fig..1 also on the same scale;

Figs. 4 and 5 are front views, substantially full size and on a reduced scale from Figs. 1, 2 and 3, showing the magnet and pole piece assembly before and after completion; and

Fig. 6 is a detail view, partly in cross section vand on a greatly enlarged scale over that of Figs. 1, 2 and 3, of the armature assembly of the device of Figs. l, 2 and 3.

Referring to the drawing, 1 is a molded composition base of insulated material adapted to re'- ceive front and rear cover plates 8 and 9 to form therewith a housing or casing for the transducer operating mechanism. The base plate vis provided with a rear extension or shank I0 for connection with a suitable tone arm (not shown), and embedded therein are the usual electrical terminal means II for the device.

The base is provided with two spaced depending leg portions vI2 and each is provided with an elongated slot or opening as receiving a clamping screw indicated at I4 for the interior mechanism. The clamping bolts also engage a portion of a rear cover I5forv the mechanism to hold said cover in place.

On the interior face of the base,l a unitary magnet structure is provided, comprising a U-shaped permanent magnet I6 and a pole piece I1 as shown more clearly in Fig. 4. The pole piece Il is provided with welding points I8 for engaging the poles of the permanent magnet I6 and is thereby spot welded as indicated in Fig. 5 to pro- I9, that is, a tight joint having low reluctance.

The construction shown further includes in the pole piece, two rectangular end portions inindicated at I3 for" 4 and 5 to provide a thin bridge portion 24 through which the air gap slot 2U is madel by cutting or milling after welding the ends to the magnet.

It will be seen that a unitary pole piece and magnet structure is thus provided in which the pole piece is welded to the magnet so as to insure a good magnetic circuit at all timeseven when the pickup device isgbeing serviced `in the field. The magnet acts as a bridge or yoke to maintain a constant air gap between the severed portions of the pole piece without utilizing a special connecting link or bridge piece as has been necessary heretofore. 'I'he thin wall bridge portion of the pole piece is furthermore reduced in cross section at the center on a taper forming the endV sections toward the center whereby thelresulting pole tips 25, Fig. 5 are tapered. The air gapis,

therefore, substantially equal in length to the .A

width of the armature tail or upper Yenol hereinafter described.

The unitary pole piece and magnet structure is provided with two tapped holes 26 to receive the clamping screws I4 through the slots i3 and permits the magnet structure 'to be adjustedon the base 'l before being finally clamped in position. A pickup coil or `winding' 2l is mounted on a flat strip of insulating material 28, which is seated against the bottom or lower .ends of the pole piece. The pole piece ends are also tapped to receive armature clamping screws 29, which serve to draw a clamping plate V3G against the mounting strip 28 as shown to return the latter in` a seated position against'the'ends of the pole piece and to clamp the ends 3|-,32 of the armature pivot shaft. YThe plate orv armature Clamp 3G is of non-magnetic material, preferably of Zinc alloy.

' Thearmature pivot shaft is integral with the armature`33 which 'extends through the coil 2l' and the space or large airi'gap 23, into the main or narrow air gap 20. The armature pivot shaft is surrounded by a spacer cushion preferably of rubber in the form of sleeves 34 whereby the armature is resiliently mounted when the clampfing screws 22 are tightened as shown in'Fig. l. The armature pivot shaft portion 32 is tapped to receive aneedleV clamping screw 35 :and extends frontly to receive the curled en 'd of a centering spring 36 which isV soldered'or otherwise suitably securedthereto as shown in Figs; l, 2Vand 6 particularly. The centering spring is bent at an angle to lie across one frontal face of the pole piece to .be clampedby a clamping piece 3l drawn inwardly to 'grip it by afbolt 38 which is threaded intoi one of the tapped openings 25 in the pole piece. S;

VIt will be noted thatl the Yarmature clamp and the coil mounting strip 28 are slotted about the clamping screw 29- to permit adjustment of the two members with the coil'Z'l and the armature 33 to center the armature tail or free end later in the air gap 20 as shown, particularly in Fig. l, before tightening the clamping member 31 by meansv of the bolt 38f Y 'Ihe opposite or rear end ofthe armature pivot shaft *isY extended to receive a block or strip Vof inertdamping material, such as Viscoloid-,a Du Pont product, indicated at 39, one end of the strip being .pressed on to the armature shaft as shown more particula'rlyinFigs. 2 and 6, with the opposite end extending upwardly to` a position wherein it isY clamped Vagainst thelbase member 7. A vclamping screw 4l)Y extendingnfthrough the base into a combined bracketY and clamping mem'-V Y is inserted through the block. The screws I4 are then tightened to hold the assembly on the base.

rThe damping block .is secured to the armature shaft by frictionV and, to provide a tight joint, the

armature shaft end is slightly heated to cause the .material to melt and How about the shaft when the armature assembly is completed as shown in Fig. 6. It will be seen that with this construction the armature spring and damping block to gether with the armature spacer cushions may be assembled as a unit.

The pickup coil or winding 21 is provided with output leads 44 `which are connected to the terminals Il, .as shown. The armature is provided With an axially extending recess for receiving a stylus or needle 45 of any suitable type and the needle is retained therein by the needle screw as indicated in Fig. 2.

A'second unit of the construction comprises the coil 27 and mounting strip 28 therefor which'may be assembled with the magnet and pole piece structure and as a completed unit is entirely removable from the base lwvithout disturbing the relation between the magnet and its pole piece and without disturbing the air gap and the flux existing in the magnetic circuit. It

will' be seen that the magnetic circuit may not be broken by accidental separation of the parts and that the air gap is permanently fixed to a predetermined width.

It will further be seen that with the construction shown, the pickup coil or winding lies within a rela'tivelyrwide airgap between magnetic poles and theV armature extends through the coil into a relatively narrow thin air gap forming the only low reluctance path for the flux since the armature clamping plate is of non-magneticrmaterial. The pickup winding, therefore-lies within the normal flux path of the magnet and only the tip ofV the armature liesY in the main or narrow air gap.` lIthas Ybeen found that this arrangement provides high fidelity in the translation of signals from and to a phonograph record in connection with an audio frequency signal circuit.

It will further be seen that an'advantage in the construction shown resides in the unitary magnet and pole piece construction wherein the magnet is U-shaped providing spaced poles and a pole piece having end portions welded tothe poles of the magnet to provide a low reluctance fiux path throughthe joint. A further advantage in this construction lies inthe fact that the magnet itself forms ayoke for the pole pieces or pole pieceto maintain a Yfixed air gap without additional bridge elements.

inthe field.

yexternal to the magnet'between vthe ends .of said pole piece, an electrical'winding within thefpole piece adjacent to said bridge portion, and an armature member extending through the winding and into said air gap. y

2. In an electro-magnetic pickup device, a unitary pole piece and magnet structure comprising a U-shaped magnet, and a pole piece member of magnetic material extending between and welded to the poles of said magnet, said member having a thin web between the poles and an air gap therein maintained fixed in width wholly by said magnet as yoke, and providing the only magnetic conducting path external to the magnet between the ends of the pole piece member.

3. In an electro-magnetic pickup device, the

, combination with a U-shaped magnet, of a unitary pole piece of magnetic material permanently united therewith at the magnet pole faces, said pole piece providing a portion adjacent each pole of the magnet of substantially the same crosssectional area as'the magnet and a thin unitary bridge portion therebetween of lesser cross-sectional area, said bridge portion constituting the only magnetic conducting element external to the magnet betweenv the ends of said pole piece and having a slot providing an air gap between said ends and said magnet thereby providing a yoke to hold said pole piece permanently with said air gap fixed.

4. In an electro-magnetic pickup device, the combination with a magnet having poles in spaced relation to each other, of a pole piece permanently united with said poles and extending therebetween, said pole piece providing endportions of relatively large cross-sectional area, one adjacent to and contacting each of said poles, and a thin unitary bridge portion of lesser cross-sectional area therebetween having a slot providing an air gap between the magnet poles, said magnet thereby providing a yoke to hold said pole piece permanently with said air gap fixed, and said bridge portion providing the only magnetic conducting member external to the magnet and between the poles thereof.

5. An electro-mechanical translating device.

`relatively small cross-sectional area providing the only magnetic circuit element external to the magnet between said end and having an air gap maintained constant in width by said magnet as a yoke.

6. An electromagnetic pickup device comprising a lU -shaped magnet, a pole piece .member of magnetic material having ends welded to the magnet poles and having 'a thin intermediate web slotted to provide a fixed air gap maintained permanent wholly by said magnet as a yoke and providing the only magnetic conducting path external to the magnet between the magnet poles, said magnet and pole piece thereby providing a unitary magnet and pole piece structure, a flat plate of nonmagnetic material engaging the pole piece ends in spaced parallel relation to said central web, a coil mounted on and secured to said plate between said plate and said central web, a vibratory armature extending through the coil and into the air gap, means providing a pivot shaft for the armature extending transversely of said coil support, a

clamping plate of non-magnetic material for engaging the armature shaft, and means for adjustably-securing saidA clamping and mounting plates to said pole piece member. f

7. An electromagnetic pickup device comprising a U-shaped magnet, a pole piece member having ends welded to the magnet poles and having a thin central web slotted to provide a xed air gap, said magnet and pole piece thereby providing a unitary magnet and pole piece structure, a at plate of non-magnetic material engaging the pole piece ends in spaced parallel relation to said central web, a coil mounted on and secured to said plate between said plate and said central web, a vibratory armature extending through the coil and into the air gap, means providing a pivot shaft for the armature extending transversely of said coil support, a centering spring for the armature secured at one end to one end of said pivot shaft, means for adjustably clamping the opposite end of the armature centering spring to one end of the ypole piece, a block of vibration damping material connected with the opposite end of said pivot shaft, a base for said pickup device,

' means for clamping said block of damping material to said base in spaced relation to its connection with the pivot shaft, and means for adjust-` ably securing said unitary magnet and pole piece structure to said base member.

8. An electromagnetic pickup device comprising a U-shaped magnet, a pole piece member having ends welded tothe magnet poles and having a thin central web slotted to provide a xed air gap, said magnet and pole piece thereby providing a unitary magnet and pole piece structure, a flat plate of non-magnetic material engaging the pole piece ends in spaced parallel relation to said central web, a coil mounted on and secured to said plate between said plate and said central web, al

vibratory armature extending through the coil and into the air gap, means providing a pivot shaft for the armature extending transversely of said coil support, means providing a resilient bearing for the pivot shaft on op-posite sides of and adjacent to the armature, the ends of the shaft projecting axially from said bearing means, a centering spring for the armature secured to one projecting end of said shaft, and a block of vibration damping material secured to the opposite projecting end of said shaft.

9. A transducer of the electromagnetic pickup type comprising in combination a base member of insulating material, a magnet and pole piece structure comprising a U-shaped permanent magnet and a rectangular pole piece bridge across the ends of said `magnet and welded thereto to provide a low reluctance magnetic circuit, said pole piece being'recessed intermediate between magnet element thereof as a yoke and having a thin severedcentral web providing pole tips, said web forming the only low reluctance flux path between the magnet poles, an electrical pickup winding lying within the normal ux path of said magnet and pole piece structure, and an armature extending through the coil and intothe air gap, said armature having a pivot shaft, an armature centering spring connected with one end of said shaft anda block of vibration damping material 10 connected with the opposite end of said shaft.

EMILE C. GUEDON. 

